Image pickup elements, such as CCDs (Charge Coupled Devices) and CMOSs (Complementary Metal Oxide Semiconductors), have been used in digital still cameras and similar imaging devices with image pickup elements that are extremely miniaturized. This miniaturization, along with the light weight that this miniaturization supports, has been accompanied by a demand that the main body of the image pickup devices and the lenses mounted in these devices and used for imaging be similarly miniaturized and made light weight.
In recent years, image pickup elements having a large number of pixels have been developed in order to achieve high image quality, and lens systems that provide even higher resolution and higher contrast performance also have been required. Moreover, there is now a requirement for an optical zoom mode capable of zooming without deterioration of the image and the development of a compact image pickup device mounted with a zoom lens has been desired.
In an image pickup device using a mounted zoom lens for imaging, the thickness of the image pickup device is largely dependent upon the size of the lens system. Particularly, in a construction in which optical elements forming the lens system are arrayed linearly in one direction without changing the direction of the optical axis, the size of the image pickup device in the thickness direction is actually determined by the length from the optical element on the object side of the lens system to an image pickup element. On the other hand, the larger the number of lens elements in the lens system, the more easily the recent requirements for a larger number of pixels and higher performance of the image pickup element may be satisfied, making it difficult to shorten the overall length of the lens system, and, in turn, making it difficult to make the entire image pickup device thinner. Accordingly, a zoom lens has been proposed that adopts a so-called reflective optical system in which the optical path of the lens system is folded in order to make the image pickup device thinner.
For example, the zoom lens described in Japanese Laid-Open Patent Application 2000-131610 is constructed with a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power, and, additionally, includes a prism having a reflective surface for folding the optical path at least approximately ninety degrees in the first lens group. This zoom lens performs zooming by moving the second lens group and the fourth lens group.
When the zoom lens described in Japanese Laid-Open Patent Application 2000-131610 is mounted in an image pickup device, its thickness as an image pickup device greatly depends on the size of the portion of the first lens group, that is used for folding the optical path, rather than the overall length of the zoom lens along the optical axis. Therefore, in order to make the image pickup device thinner, it is preferable to miniaturize the first lens group. In the zoom lens of Japanese Laid-Open Patent Application 2000-131610, the lens element on the object side of the zoom lens is especially large and there is room for improvement in terms of miniaturization. Additionally, the development of a zoom lens that relaxes requirements for minimizing the diameter of the lens element on the object side in achieving miniaturization and that is suitable for mounting in an image pickup device in order to make it thinner, has been desired.